1. Field of the Invention
The present invention relates to surface acoustic wave devices, and more particularly to an acoustic periodic structure having a corrugated surface capable of supporting a horizontally polarized shear surface wave.
2. Background of the Invention
The attractions of surface acoustic wave (SAW) components for use in oscillators and frequency filters have been reported extensively since 1974. SAW devices are useful over the range 10 to 1000 MHz, although below 100 MHz SAW devices become inconveniently large. This is because SAW devices require long grating reflectors having 400-500 or more elements spaced .lambda./2 apart.
By using the analogy between Maxwell's equations, applied to the propagation of a TM-surface wave along an infinite corrugated surface, and the acoustic field equations, a new type of horizontal shear surface wave is represented. (See Auld et al, "Horizontal Shear Surface Waves on Corrugated Surfaces," Electronics Letters Vol. 12, No. 24, pg 650 Nov. 25, 1976). The acoustic variable analogous to H.sub.x is the x component of particle displacement velocity which satisfies the ordinary wave equation in two-dimensional isotropic problems. At traction-free boundaries parallel to the x-axis the normal derivative of V.sub.x is required to be zero. This shows that the electromagnetic problem of propagation of a TM-type surface wave along an infinite corrugated surface has an exact acoustic analog to a free isotropic elastic plate with a uniform grating of slots cut into the upper surface.
Although most surface acoustic wave research has been concerned with Rayleigh waves, horizontally polarized surface waves are not unknown. Such waves exist on piezoelectric and magnetostrictive substrates, and also on layered isotropic substrates (Love waves). The waves considered here are unique, however, in that they have a propagation velocity much slower than the bulk shear wave velocity and exhibit an upper cut-off frequency. At this point the period of the grating may be significantly less than .lambda..sub.shear /2.